In general, there are two complementary approaches to manufacturing an object: additive manufacturing and subtractive manufacturing.
Additive manufacturing involves aggregating material to form the desired object. In contrast, subtractive manufacturing involves removing material to form the desired object. In practice, many objects are manufactured using a combination of additive and subtractive techniques.
A form of additive manufacturing—colloquially known as “three-dimensional (3D) printing”—is the subject of intense research and development because it enables objects with complex geometries to be manufactured without molds or dies, although molds or dies may be used to support the manufacturing process in some cases. Furthermore, 3D printing enables the mass customization of objects with different dimensions and characteristics.
In at least one 3D printing technique, the object is built by providing a thermoplastic filament to a deposition head on a robotic arm. The thermoplastic filament comprises a continuous tow of carbon fibers that is impregnated with thermoplastic. The deposition head heats and deposits the thermoplastic filament as one or more runs of material, via a roller. After it is deposited, each run quickly solidifies and fuses with the runs that it touches. The technique thereby builds a three-dimensional object from a mathematical model of the object.
In order to build the object to within the tolerances specified in the model, the deposition head must be continuously provided with the thermoplastic filament at a known tension, or at a minimal tension so that it can be drawn by the deposition head at a known speed. When tension is sufficiently reduced, a downstream system, such as the deposition head, is able to control the speed and tension of the filament directly, and the formation of uncontrolled loads in the filament is prevented. Further, if tension is not sufficiently reduced, upstream and downstream impedances compete to reduce predictability and create a variety of problems. For example, if the spool inertia is sufficiently great and the feed motor in the deposition head is sufficiently powerful, the filament could break, given a slight mismatch in spool and deposition feed rates. Alternatively, if a feed motor is insufficiently powerful, part quality could be reduced as tension in the filament is higher than expected and the achievable feed rate may be lower than what the deposition head's travel speed requires. Various other scenarios can also result in unpredictable interplay between components of the system, thereby reducing the quality of parts printed in such a system.
Accordingly, speed or tension at a filament deposition head should be decoupled from speed or tension or any other loads in an upstream system. Providing a known tension, such as a minimal tension, or a controllable speed results in predictable filament uptake by a deposition head, as well as known characteristics of the filament during deposition. The tension or speed must further be kept substantially constant, or controllable by the deposition head, regardless of variations in tension or speed of the filament as provided from a source. Such variations in the source may be due to inertia in a source material spool, a mismatch between the speed of a motor at source of the filament, such as a spool, with the speed at which the filament is drawn by the deposition head, or friction at some point within the system. Because such a tension or speed mismatch may result in insufficient or excess filament being provided to the system, the filament must be accumulated and placed somewhere. The accumulated reserve material must be kept in a location where there is a safe margin of additional space so that excess material coming from the source or the deposition head can be temporarily stored in the accumulator.
A filament source may ideally provide filament to be drawn by a deposition head without substantial tension, in which case the deposition head can control all downstream properties of the filament, including speed and tension. This may be, for example, by manually feeding filament to the deposition head or by manually unspooling filament and providing it to the deposition head from a pile or the like. However, this dramatically limits the volume of filament that can be provided, or the speed at which the process can be implemented. Typically, filament is provided on a spool, and the spool itself may generate friction or inertia that results in varying speeds or tensions in the line. This may be addressed by providing a filament source that may be consistently driven with a motor such that it matches the speed at which filament is drawn by a deposition head. However, even in such an embodiment, mismatches are possible where there are sudden changes at the deposition head, such as a cut after a run of material, or where a change of speed occurs at different rates, such as due to inertia at the filament source.
Traditional applications that require a thread or filament provided at a known tension, or at a controllable or known rate despite variations in a source, in other contexts may rely on a dancer tension control system. Such a dancer may be an extra roller that is used to vary the length of a path between two points in order to account for variations in a source tension or a speed mismatch by taking up excess thread or filament. Such a dancer isn't practical when the source thread or filament is relatively stiff, since it would require substantial baseline tensioning and a large amount of space. Further, because of the dimensions, inertias and tensions required at the dancer roller, such a system for a stiff filament may require active control of the roller.
What is needed is a system for managing tension or speed of a filament provided to a deposition head for use in 3D printing without the use of a dancer roller. There is a further need for such a system that manages takeup of excess filament and is relatively easy to load without at least some of the disadvantages in the prior art.